1. Field of the Invention
The present invention relates to an optical device, and an optical circuit or an optical coupler which use the optical device, and integrated circuit comprising the optical circuit. For example, the present invention relates to an optical device used for an optical circuit or an optical coupler utilizing multimode interference.
2. Related Art of the Invention
A self imaging phenomenon of multimode interference (MMI) in a rectangular multimode waveguide involves a reduced loss, is suitable for miniaturization, and allows easy processing. Thus, the rectangular multimode waveguide is utilized as an optical circuit and an optical coupler for an optical multiplexer and demultiplexer.
With an increased demand for communication, there has been an increased requirement not only for an increase in the transmission rate and channel number of optical devices but also for miniaturization and integration thereof.
However, a rectangular multimode waveguide with a reduced size and multiple channels for input and output offer an increased number of high-order modes and an increased pulse width spread. This disadvantageously prevents efficient transmission.
That is, mode group delay dispersion in the rectangular multimode waveguide causes a group delay difference between modes such that the group delay increases consistently with the order of the mode, that is, from low-order mode to high-order mode. Thus, an increase in the number of high-order modes increases the pulse width spread, caused by the group delay difference. Transmission rate is limited by the pulse width spread. Consequently, the increased number of high-order modes prevents efficient transmission.
Now, with reference to FIG. 11, a mechanism will be described about the problem of the increased the number of high-order modes. FIG. 11 shows a schematic diagram of a conventional rectangular multimode waveguide with multiple channels.
In FIG. 11, a plurality of input side single-mode waveguides 32 arranged in parallel so as to correspond to the multiple channels are connected to an input facet of a conventional rectangular multimode waveguide 31 so that connection positions are arranged on the input facet of the rectangular multimode waveguide 31 in a width direction thereof. An external light source 34 is connected to each of the input side single-mode waveguide 32. An output side single-mode waveguide 33 is connected to a width-wise central position of an output facet of the rectangular multimode waveguide 31.
For example, on the rectangular multimode waveguide 31 in FIG. 11, the lights inputted from the input side single-mode waveguide 32 interfere with each other, in other words high-order modes generated, and coupled. The coupled light is output from the output side single-mode waveguide 33. However unnecessary high-order modes are generated at the same time. In the case of the input side single-mode waveguide 32 are six as FIG. 11, 6th-order mode and lower order mode than 6th-order mode are necessary for the inputted lights coupled. However 7th-order mode and higher order modes than 7th-order mode which are unnecessary for coupling are generated at the time. When the unnecessary high-order modes are generated, the width of the pulse of the light outputted from the output side single-mode waveguide 33 is spread. Even if the 1st-order mode is not mixing on the input side single-mode waveguide 32 and only the 0th-mode is inputted to the rectangular multimode waveguide 31, the unnecessary high-order modes for coupling are generated.
Moreover for example, there is another problem that the unnecessary 1st-order mode is generated on the input side single-mode waveguide 32 which 0-th order mode should input the rectangular multimode waveguide 31. The mechanism is also reported in C. Vazquez, C. Aramburu, M. Galarza, and M. Lopez-Amo, “Experimental assessment of access guide first-order mode effect on multimode interference couplers,” Opt. Eng. 40(7), pp 1160-1162, 2001.
As shown in FIG. 11, really speaking, the distance between the plurality of input side single-mode waveguides 32, connected to the rectangular multimode waveguide 31, decreases with increasing number of channels and decreasing size of the waveguide. Thus, directional coupling 35 causes mode coupling between the adjacent input side single-mode waveguides 32 to generate the 1st-order mode.
When light from the external light source 34 enters the input side single-mode waveguide 32 and if off-axis incidence 36 occurs, the 1st-order mode is also generated. And, an increase in the number of channels increases the frequency with which the 1st-order mode is generated by the off-axis incidence 36. The 1st-order mode generated by the off-axis incidence 36 attenuates as the light propagates through the input side single-mode waveguide 32. However, owing to a reduced optical path length resulting from the reduced size of the device, the 1st-order mode enters the rectangular multimode waveguide 31 before being completely attenuated.
Thus, with the multiple channels, the directional coupling 35 and off-axis incidence 36 cause not only the 0th-order mode but also the 1st-order mode to be input to the rectangular multimode waveguide 31. As a result, this increases the number of high-order modes in the rectangular multimode waveguide 31.
As described above, there are two problems. The 1st problem is that the unnecessary high-order modes are generated in the rectangular multimode waveguide 31, and the 2nd problem is that the unnecessary 1st-order mode is generated on the input side single-mode waveguide 32.
These two problems will be described below with reference to FIG. 12.
FIG. 12 shows output waveforms (waveform envelopes: theoretical calculation results, waveforms: FDTD (Finite-Difference Time-Domain) simulation results) observed in the rectangular multimode waveguide 31 under an MMI self imaging condition using a 1st-mode input light quantity rate (R1) and input positions (Ch0, ±Ch1, ±Ch2, ±Ch3: Ch0 is the width-wise central position of the input facet of the rectangular multimode waveguide 31) as variables. A waveform shown at the top of FIG. 12 is an input waveform input to each of the channels. The attenuation indicated in the FDTD simulation results is faster than that indicated in the theoretical calculation results because a model that more significantly attenuates harmonic components with a more significant group delay in a propagation mode is used in order to reduce time for FDTD calculations and a load on a computer.
When you look at the case of that only 0th-order mode is inputted and 1st-order mode is not inputted (the case of “R1=0”) with reference to FIG. 12, you will be recognize that the output waveform spreads corresponding to the input waveform about the light inputted to any position. This is the pulse width spread caused by the unnecessary high-order modes generated in the rectangular multimode waveguide 31.
Furthermore, the multiple channels and the miniaturization and integration reduce the diameter of the core of the input side single-mode waveguide 32, connected to the rectangular multimode waveguide 31, while increasing the number of inputs provided closer to width-wise ends of the input facet of the rectangular multimode waveguide 31. This also increases the number of unnecessary high-order modes in the rectangular multimode waveguide 31.
Furthermore, if you look at different output waveforms of the 1st-mode input light quantity rate (R1) with reference to FIG. 12, you will be recognize that the input of the 1st-order mode to the rectangular multimode waveguide 31 laterally unbalances the MMI to cause an output variation, further increasing the pulse width spread. The variation of the output waveforms with the 1st-mode input is caused by the unnecessary 1st-order mode generated on the input side single-mode waveguide 32.
In view of the above-described conventional problems, an object of the present invention is to provide an optical device, an optical coupler and an integrated circuit that enable a reduction in the pulse width spread caused by the mode group delay dispersion in spite of multiple channels and the reduced size of the device.